The present invention relates generally to methods whereby heat fluxes on process tubes within process heaters may be manipulated so as to be more equal circumferentially. The methods of the invention are especially well suited for use in coke sensitive fired heaters employed in the petroleum refining industry, such as coker units, vacuum units, crude heaters, and the like.
Most coker sensitive heaters or furnaces, such as coker, vacuum and crude heaters, are so-called single fired units which employ a source of combustion generally centrally of an array of process tubes. The process tubes are thus typically positioned closely adjacent the refractory wall of the heater which results in uneven circumferential heat flux distribution. That is, circumferential segments of the tube adjacent the combustion element of the heater is typically hotter than the circumferential segment of the tube adjacent the refractory wall of the process vessel.
The heat flux on the hotter fired side of the tube results in higher tube metal temperature as compared to the refractory wall side of the tube. A higher coking deposition rate internally of the tube at the hotter fired side thereof is the net result of such uneven circumferential heat flux deposition. Such unequal internal circumferential coking also leads to premature disadvantageously high pressure drop through the tube and/or a disadvantageously high temperature at the exterior surface of the tube (i.e., since the coking on the internal tube surface acts as an insulator). Consequently, reduced operational run lengths for the fired heaters ensue. For example, a typical coker unit requires decoking every six to nine months, with some coker units requiring decoking every three months.
There is also unequal heat fluxes which exist within the process heater itself which can result in relatively uneven coking from one tube section to another. Thus, some tubes or tube sections may be closer to the combustion source as compared to other tubes or tube sections within the process heater. Those tubes more remote from the combustion source (e.g., those tubes near the top of the heater when the combustion source is at the heater bottom) may have circumferential segments of the tube which exhibit a lesser heat flux as compared to similar circumferential segments of tubes closer to the combustion source even though the circumferential segments are oriented so as to face the heat generated by the combustion source.
It would therefore be highly desirable if process tubes or tube segments within fired vessels could be imparted with a more uniform circumferential heat flux distribution. It would also be desirable if heat flux within the process heater could be more equally redistributed by virtue of providing different tubes and/or tube sections with predetermined different, but locally substantially uniform, circumferential heat flux distribution. It is therefore towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is directed toward methods for providing more equal heat flux distribution about an exterior circumferential surface of at least one section of a process tube within a process heater, and to such process tubes on which a more equal circumferential heat flux distribution has been imparted. More specifically, according to the present invention, there is provided on at least one circumferential segment of at least one exterior circumferential surface section of the process tube, a coating of a material having a selected thermal emissivity and/or thermal conductivity which is different from the thermal emissivity and/or thermal conductivity of another circumferential segment of the same exterior circumferential surface section of the process tube. In such a manner, a more equal thermal conductance about an entirety of the exterior circumferential surface section of the process tube is established as compared to the thermal conductance thereabout in the absence of the coating, thereby resulting in a more equal heat flux distribution circumferentially on the tube section.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.